The invention relates to a method for measuring the force interaction caused by a sample, in particular for a scanning tunneling microscope, and to a scanning tunneling microscope.
The scanning tunneling microscope (STM) and the operating principle thereof are generally known and understood. Because of special design features, the scanning tunneling microscope achieves lateral resolutions in the range of up to 0.1 angstrom and thus allows nanostructures to be visualized by way of the topography and/or local electronic density of states in the region of the valence electrons.
The atomic force microscope (AFM) utilizes the force interaction between a tip and a sample to map the sample surface. Contrary to the STM, a variety of fundamentally different types and operating modes exist with the AFM, which differ from each other in terms of the manner in which the force is detected. This variety reflects attempts to improve the resolution of the AFM into the range of atomic resolution, which has disadvantageously been extraordinarily difficult.
In all instances of atomic force microscopy, the force, or a variable derived therefrom, is used as the control variable for operating a feedback loop. The most common conventional operating modes are:
a) static AFM:
A tip, which is provided at the end of a soft bending beam in the form of what is known as a cantilever, is guided over the sample, with contact (contact mode), or in a contactless manner (non-contact mode), wherein the force of interaction is measured by way of the deflection of the cantilever. The common type of the static AFM is the bending beam AFM, usually with optical detection of cantilever deflection.
b) Tapping Mode:
This mode can be employed in different types. A tip oscillates over a surface and repeatedly comes in contact therewith.
c) Contactless Mode:
Here, the interaction between the tip and sample is measured without the two coming in contact with each other. The sensor is caused to oscillate often so as to prevent what is known as a jump to contact. This is then also referred to as a dynamic AFM. The dynamic AFM employs four different force sensors: cantilever, QPIus sensor, needle sensor and tuning fork, wherein these can be roughly divided into two categories depending on whether they are driven mechanically or electrically. What is measured is the frequency shift or amplitude shift of the force sensor. An important advantage of all the aforementioned AFM designs is the option of being able to conduct measurements without limitation, even on non-conducting samples.
Over the last few years, a wide variety of efforts have been made to improve the lateral resolution of the AFM toward (sub)atomic or submolecular resolution. Dynamic atomic force microscopy, which in comparison with the STM has the disadvantage of necessitating high apparatus-related costs, has so far been the most successful on this path.
A disadvantage of scanning tunneling microscopy is also the lack of chemical sensitivity, which is to say that STM does not allow chemical species to be identified, which means that, while molecular objects and surface structures in the lateral size range down to less than one angstrom can be imaged, they cannot be identified, chemically or otherwise.
In summary, the conventional STM offers lower experimental complexity than the high-resolution dynamic AFM, but has the disadvantage of providing no chemical information. The high-resolution dynamic AFM, in turn, offers more detailed information about the sample surface than the STM, including the chemical kind, but it has the disadvantage of being associated with greater experimental and apparatus-related cost.